Hydrocarbon recovery in naturally fractured reservoirs is typically very low. In medium to high-permeability naturally fractured carbonate reservoirs, in particular, oil recovery is often less than 15% of the calculated oil-in-place. In this case it is generally difficult to recover the oil from the matrix due to the easier flow path offered to the oil in the naturally fractures. Water flooding is often used to displace oil from the carbonate reservoir but again this is generally only effective in the presence of the higher permeability natural fractures.
As many carbonate oil reservoirs are mixed or oil-wet, spontaneous imbibition of the water from the water flood does not improve recovery from the bypassed natural fractures and matrix portion of the reservoir. One of the basic problems lies in increasing the sweep efficiency of water flooding. Injected water typically finds the production wells quickly due to the channeling of the water through the high permeability natural fracture system, leaving behind the bulk of the hydrocarbon in the matrix.
Even in cases where the imbibition or water flooding is successful, the process is very slow and/or water production or cycling very high. The process is hampered for example by mixed wettabilities.
The first use of semi-permeable membranes to create osmotic pressure for oilfield application has been previously described in the co-owned U.S. Pat. No. 6,069,118. The patent describes the use of a chemical potential gradient or osmotic pressure gradient to remove fracturing fluid from an artificially created fracture and thereby increasing the effective length of the created fracture.